mtd: nftl: Remove VLA usage

On the quest to remove all stack VLAs from the kernel[1] this changes
the check_free_sectors() routine to use a kmalloc()ed buffer instead
of a large VLA stack buffer.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/mtd/inftlmount.c

@@ -334,28 +334,37 @@ static int memcmpb(void *a, int c, int n)
 static int check_free_sectors(struct INFTLrecord *inftl, unsigned int address,
 	int len, int check_oob)
 {
-	u8 buf[SECTORSIZE + inftl->mbd.mtd->oobsize];
 	struct mtd_info *mtd = inftl->mbd.mtd;
 	size_t retlen;
-	int i;
+	int i, ret;
+	u8 *buf;
 
+	buf = kmalloc(SECTORSIZE + mtd->oobsize, GFP_KERNEL);
+	if (!buf)
+		return -1;
+
+	ret = -1;
 	for (i = 0; i < len; i += SECTORSIZE) {
 		if (mtd_read(mtd, address, SECTORSIZE, &retlen, buf))
-			return -1;
+			goto out;
 		if (memcmpb(buf, 0xff, SECTORSIZE) != 0)
-			return -1;
+			goto out;
 
 		if (check_oob) {
 			if(inftl_read_oob(mtd, address, mtd->oobsize,
 					  &retlen, &buf[SECTORSIZE]) < 0)
-				return -1;
+				goto out;
 			if (memcmpb(buf + SECTORSIZE, 0xff, mtd->oobsize) != 0)
-				return -1;
+				goto out;
 		}
 		address += SECTORSIZE;
 	}
 
-	return 0;
+	ret = 0;
+
+out:
+	kfree(buf);
+	return ret;
 }
 
 /*

drivers/mtd/nftlmount.c

@@ -272,28 +272,37 @@ static int memcmpb(void *a, int c, int n)
 static int check_free_sectors(struct NFTLrecord *nftl, unsigned int address, int len,
 			      int check_oob)
 {
-	u8 buf[SECTORSIZE + nftl->mbd.mtd->oobsize];
 	struct mtd_info *mtd = nftl->mbd.mtd;
 	size_t retlen;
-	int i;
+	int i, ret;
+	u8 *buf;
 
+	buf = kmalloc(SECTORSIZE + mtd->oobsize, GFP_KERNEL);
+	if (!buf)
+		return -1;
+
+	ret = -1;
 	for (i = 0; i < len; i += SECTORSIZE) {
 		if (mtd_read(mtd, address, SECTORSIZE, &retlen, buf))
-			return -1;
+			goto out;
 		if (memcmpb(buf, 0xff, SECTORSIZE) != 0)
-			return -1;
+			goto out;
 
 		if (check_oob) {
 			if(nftl_read_oob(mtd, address, mtd->oobsize,
 					 &retlen, &buf[SECTORSIZE]) < 0)
-				return -1;
+				goto out;
 			if (memcmpb(buf + SECTORSIZE, 0xff, mtd->oobsize) != 0)
-				return -1;
+				goto out;
 		}
 		address += SECTORSIZE;
 	}
 
-	return 0;
+	ret = 0;
+
+out:
+	kfree(buf);
+	return ret;
 }
 
 /* NFTL_format: format a Erase Unit by erasing ALL Erase Zones in the Erase Unit and